Moderately High Obliquity Promotes Biospheric Oxygenation

We first consider the results from a range of obliquity scenarios (0°−45°) with present-day Earth ocean phosphate inventory and RLS to determine planetary obliquity's effect on export POC, the flux of organic carbon produced by photosynthesis that evades remineralization in surface waters and sinks to the deeper ocean—leaving behind oxygen. We additionally quantify the response of sea-to-air oxygen flux resulting from photosynthesis. In these obliquity scenarios (ranging from 0° to 45°), annual total global export POC generally increases with increasing obliquity, with export POC increasing by 35% between the 0° and 45° obliquity scenarios. However, export POC reaches a minimum at 15° obliquity (Figure 2(a), magenta line), decreasing by 5% between the 0° and 15° obliquity scenarios.

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We also see that global oxygen sea-to-air flux (${F}_{{{\rm{O}}}_{2}}$) increases with obliquity. There is a twofold increase in ${F}_{{{\rm{O}}}_{2}}$ between the 0° obliquity and 45° obliquity scenarios (Figure 2(b), magenta line). While a portion of this increase in ${F}_{{{\rm{O}}}_{2}}$ from the ocean to the atmosphere is a consequence of increased photosynthesis, as evidenced by the increase in export POC values with increasing obliquity, the ratio of ${F}_{{{\rm{O}}}_{2}}$ to POC also increases with higher obliquity (Figure 2(c), magenta line). This increase in ${F}_{{{\rm{O}}}_{2}}$:POC in moderately high obliquity scenarios suggests that physical effects caused by moderately high obliquity also contribute to the increase in ${F}_{{{\rm{O}}}_{2}}$ with higher obliquity.

We then simulate oceans with a range of phosphate inventories and planetary obliquity values. Increasing phosphate inventory in our simulations has an ∼1:1 effect on increasing export POC (Figure 2(a)). The phosphate inventory also affects the biospheric response to obliquity. The biosphere is relatively insensitive to obliquity when phosphate is scarce (0.25–0.5× POL), but export POC becomes increasingly sensitive to obliquity under phosphate replete conditions (2–4× POL).

This same trend is seen in ${F}_{{{\rm{O}}}_{2}}$ values as well (Figure 2(b)). At low phosphate inventory levels, ${F}_{{{\rm{O}}}_{2}}$ is relatively unaffected by obliquity. However, ${F}_{{{\rm{O}}}_{2}}$ increases more strongly with obliquity when phosphate is more abundant. At high phosphate inventory (2–4× POL), increasing obliquity from 0° to 45° yields greater that 2× ${F}_{{{\rm{O}}}_{2}}$, an effect similar to doubling the phosphate inventory.

The ratio of ${F}_{{{\rm{O}}}_{2}}$ from sea to air to unit export POC experiences a similar twofold increase with obliquity increase from 0° to 45° to that caused by a doubling of phosphate inventory (Figure 2(c)). Unlike for ${F}_{{{\rm{O}}}_{2}}$ values, scenarios at every phosphate inventory value experienced a doubling in ${F}_{{{\rm{O}}}_{2}}$:POC ratio with an obliquity increase from 0° to 45°.

As POC sinks through the water column, nutrients are made available again for metabolic reactions such as photosynthesis through remineralization at depth and transport back to the photic zone. Remineralization is the first step in this process and occurs throughout the water column. The RLS affects the vertical distribution of nutrients in the water column and the timescale for their return to the surface. When RLSs are longer, nutrient-containing particles are able to settle deeper into the water column on average before they are processed and made available for transport back to the surface, leading to greater accumulation of nutrients at depth.

Simulations with shallower RLSs displayed increased POC export and biosphere activity (Figure 3(a)). The increase in POC export with shallower remineralization occurs because nutrients do not settle as far into the deep ocean and nutrients are more readily returned to the surface via wind-driven upwelling.

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We find that export POC only slightly increases with obliquity for experiments with longer RLSs. However, export POC increases with higher obliquity for scenarios with shorter RLSs (0.25–0.5× present ocean levels, or POL, Figure 3(a)). ${F}_{{{\rm{O}}}_{2}}$ increases by ∼3–4× with increasing obliquity and decreasing RLS in all our sensitivity experiments at POL phosphate inventory (Figure 3(b)). Moderately high obliquity is a stronger driver of ${F}_{{{\rm{O}}}_{2}}$ increase than RLS decrease, as halving RLS causes an increase in ${F}_{{{\rm{O}}}_{2}}$ of approximately 50%, while increasing obliquity from 0° to 45° results in greater than double ${F}_{{{\rm{O}}}_{2}}$ (≥100% increase).

The ratio of ${F}_{{{\rm{O}}}_{2}}$ to export POC increases with increasing obliquity (Figure 3(c)) for all simulated RLSs. This result also supports our assertion that physical changes caused by increasing obliquity are partially responsible for the observed increase in ${F}_{{{\rm{O}}}_{2}}$.

Additionally, we find that while ${F}_{{{\rm{O}}}_{2}}$ and POC alone decrease with deeper RLSs, ${F}_{{{\rm{O}}}_{2}}$:POC ratio increases with deeper RLSs. ${F}_{{{\rm{O}}}_{2}}$ and export POC decrease with increasing RLS, as when the RLS is deeper, nutrients sink to deeper levels in the water column and decrease the amount of nutrients available for the surface biosphere. A decrease in photosynthesis due to a reduction of nutrients then leads to a decrease in oxygen production, and by extension ${F}_{{{\rm{O}}}_{2}}$ decrease. However, when we look at the ${F}_{{{\rm{O}}}_{2}}$:POC ratio, we see this ratio increase with deeper remineralization depths. Although both ${F}_{{{\rm{O}}}_{2}}$ and POC decrease with increasing RLS, ${F}_{{{\rm{O}}}_{2}}$:POC increases because deeper remineralization results in less near-surface oxygen consumption.

We ran a series of experiments in which we covaried obliquity, RLS, and phosphate inventory to explore potential synergistic effects introduced through covariance of these variables. Such effects could have profound implications for the potential for oxygen buildup on Earth-sized exoplanets.

The general trends for export POC, ${F}_{{{\rm{O}}}_{2}}$, and ${F}_{{{\rm{O}}}_{2}}$:POC ratio in response to increasing obliquity, phosphate inventory, and RLS previously discussed in Sections 3.1, 3.2, and 3.3 continue to hold true in the covarying simulations (Figures 4–6). Both export POC and ${F}_{{{\rm{O}}}_{2}}$ increase most strongly in response to increasing phosphate inventory, with second-order increases driven by decreasing RLS and increasing obliquity. When we increase phosphate inventory and decrease RLS in tandem, a synergistic effect is present. We find that scenarios with increased phosphate inventory and decreased remineralization depth yield >2× export POC and ${F}_{{{\rm{O}}}_{2}}$ levels than the sum of two independent scenarios with increased phosphate inventory and decreased RLS.

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The general increase in export POC with higher obliquity is driven by increased nutrient availability. As discussed earlier, Olson et al. (2020) predicted that increased seasonal deepening of the mixed layer depth with increasing obliquity during each hemisphere's winter would lead to increased nutrient cycling. This increased seasonal deepening of the mixed layer depth occurs with increasing obliquity in our scenarios as well (Figure 7(a)).

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Seasonal deepening of mixed layer depth in the winter hemisphere intensifies with increasing obliquity because as obliquity increases, the winter hemisphere receives less incident insolation. Reduced incident insolation leads to colder sea surface temperatures and weakened water density stratification, resulting in a deeper mixed layer depth. Deeper mixed layer depths allow for increased entrainment of nutrients from depth, resulting in higher surface phosphate concentrations (Figure 7(b)).

Varying seasonal nutrient availability significantly affects biological primary productivity throughout the year. In the 0° obliquity scenario, POC export rates do not significantly vary throughout the year owing to the lack of seasons (the slight variation between seasons displayed is due to the planet's nonzero eccentricity). However, for obliquity scenarios of 15° or higher, spring has the highest rate of POC export, followed by winter (Figure 7(c)). In the 45° obliquity scenario, spring export POC increases by approximately 2.5× in comparison to the 0° obliquity scenario, while winter export POC increases by a factor of two when compared to the 0° obliquity scenario. In the nonzero-degree obliquity scenarios, fall and summer have the lowest rates of POC export with summer POC export at slightly higher rates than in the fall.

POC export in the winter exceeds POC export in summer and fall owing to enhanced nutrient inventories in the surface ocean (Figure 7(c)). These nutrient rich waters are dredged up by the deepened mixed layer in these moderately high obliquity scenarios. However, although winter surface waters have an enhanced nutrient inventory, these nutrients cannot be entirely utilized owing to reduced insolation during the winter. This inhibits biological activity and therefore limits POC export until the spring. When winter ends, a "spring bloom" of heightened biological activity occurs when incident insolation begins to increase again after the dark winter season and provides residual nutrient-rich waters with light. Biological activity increases dramatically, fueled by both sufficient light and abundant nutrients, and leads to the observed maximum POC export in spring (Figure 7(c)). As a result of the deepened mixed layer and enhanced winter nutrient availability, the increases in seasonal export POC for moderately high obliquity scenarios lead to the observed increase in annual export POC with obliquity.

Though light and temperature do have significant effects on POC export in general, in our simulations these effects are negligible compared to the effect of nutrient availability. As obliquity increases, light is more evenly distributed over the planetary surface on annual average, but the total amount of light the planet's surface receives does not change, and so light cannot be the cause of the observed increase in annual export POC. Moderately high obliquity increases the amount of incident irradiation on a hemisphere during the summer months and decreases it during the winter months. One might then expect an increase in export POC during the summer and a decrease in the winter, which is not observed in our results owing to the overriding effect of nutrient availability. Additionally, in cGENIE's temperature-dependent export scheme, export POC increases with increasing temperature. One may then expect that larger temperature extremes due to increased seasonality would lead to increased export POC in the summer and decreased export POC in the winter. However, as we instead see that winter export POC surpasses summer export POC (Figure 7(c)), we conclude that nutrient availability is the dominant factor affecting export POC.

We find that both export POC and sea-to-air oxygen flux increase with increasing obliquity (Figures 2–5). This increase in ${F}_{{{\rm{O}}}_{2}}$ is a direct consequence of increased biological activity; as biological activity increases, there are more photosynthetic organisms performing photosynthesis, resulting in an increase in biogenic oxygen production. An interesting consequence of increased obliquity, however, is the effect of increasing obliquity on annual global oxygen flux per unit export POC. This increase implies that increased oxygen flux cannot be solely attributed to the increase in export POC.

If the increase in ${F}_{{{\rm{O}}}_{2}}$ with increasing obliquity was simply due to the increase in export POC with obliquity, the ${F}_{{{\rm{O}}}_{2}}$:POC ratio would be constant across all considered planetary obliquity values, as produced oxygen would scale proportionally with increased primary productivity, which it is not (Figure 2). An increase in ${F}_{{{\rm{O}}}_{2}}$:POC implies that physical effects are likely amplifying the increase in ${F}_{{{\rm{O}}}_{2}}$ with increasing obliquity.

The physical effects responsible for amplifying ${F}_{{{\rm{O}}}_{2}}$ increase with higher obliquity are decreased sea surface ice coverage and decreased solubility of oxygen in warmer water. Higher obliquity results in a more even distribution of insolation over the planetary surface. This leads to a decrease in the global average percentage of sea-ice coverage as obliquity increases (Figures 8(a), (c), (e), (g)) and warmer waters due to higher incident irradiation at the poles. As the sea-ice coverage decreases owing to the increase in obliquity and increase in ocean water temperature, a corresponding increase in oxygen flux from those now uncovered areas at high latitudes occurs (Figures 8(b), (d), (f), (h)). The resulting increase in oxygen flux occurs because (1) without ice coverage, these areas of open ocean are now able to exchange oxygen with the atmosphere with no physical barrier impeding gas exchange, and (2) oxygen is less soluble in warmer water (e.g., Wanninkhof 2014). Therefore, higher photosynthesis rates at the poles lead to enhanced sea-to-air fluxes of biogenic oxygen owing to the combined effects of increased surface area available for gas exchange and warmer ocean temperatures.

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Atmospheric oxygen may be a remotely detectable signature of life on other planets (e.g., Meadows et al. 2018). Although current telescopes are not designed to detect biotic levels of oxygen in exoplanet atmospheres (Lustig-Yaeger et al. 2019), next-generation telescopes such as the LUVOIR-like observatory recommended by the Astro2020 Decadal Survey would be able to detect biotic levels of oxygen in exoplanet atmospheres depending on cloud conditions and exposure times (Wang et al. 2018). As we have shown in our experiments, moderately high obliquity planets have higher annual global oxygen fluxes from sea to air than their low-obliquity equivalents (Figures 2(b), 3(b), 5(a)–(d)). Therefore, biospheres on moderately high obliquity exoplanets may be easier to detect than on their equivalent low-obliquity counterparts owing to their higher atmospheric oxygenation potential.

Oxygen is not the only potential biosignature that is enhanced by moderately high obliquity. Olson et al. (2018) explored the potential for the use of seasonality as a biosignature, in which the seasonal increase and decrease of biogenic gases such as CO₂, CH₄, O₂, and O₃ due to seasonal variation in insolation could be detectable by a LUVOIR-like instrument and indicate the presence of an active biosphere. As we have seen here on Earth, CO₂ and O₂ oscillate seasonally owing to changes in the balance of photosynthesis (draw down CO₂, make O₂) and respiration (release CO₂, consume O₂) (Keeling et al. 2001). While Earth has relatively moderate seasonality, exoplanets with higher obliquities would have more intense seasonality with large changes in temperature. As seasonal changes in the environment manifest as changes in atmospheric composition, larger seasonal differences may result in a larger, more detectable atmospheric signal (Olson et al. 2018).

While aquatic life is protected from seasonality owing to the high heat capacity of water, land environments may experience extreme seasonality on moderately high obliquity planets. This could be a problem because terrestrial microbes and ectothermic organisms are unable to regulate their own body temperatures (e.g., Hochachka & Somero 1977; Wright & Cooper 1981; Cossins & Bowler 1987; Huey & Kingsolver 1989) and therefore would be unlikely to survive the temperature extremes on land resulting from increased seasonality (similar to predictions by Paaijmans et al. 2013 of exotherm response to temperature variations caused by climate change on Earth). Additionally, while evolved terrestrial life that self-regulates body temperature (endotherms) can survive more extreme temperatures than ectotherms (e.g., Scholander et al. 1950; Irving & Krog 1954; Heinrich 1977; Lovegrove et al. 1991; McNab 2002), many adaptations to either extremely cold or hot environments are incompatible, such as fur or blubber.

Intensified seasonality does not have to be a death sentence for terrestrial life on moderately high obliquity planets, however. Multiple microbial species and a few multicellular species have been discovered living in extreme environments on Earth, including those at extremely cold and hot temperatures (e.g., Cavicchioli 2002). Life evolving on a planet with extreme seasons could potentially adapt to both extreme warm and cold climates and ultimately be more resilient against sudden environmental changes or mass extinctions.

Another potential problem for the origin and evolution of life on moderately high obliquity planets is water loss. Kang (2019b) found that high-obliquity planets have a much higher seasonal stratospheric water vapor abundance in comparison to equivalent low-obliquity planets, which may allow more water vapor to be lost owing to photodissociation and hydrodynamic escape of hydrogen to space. This potential for increased water loss suggests that liquid water on high-obliquity planet surfaces could be lost quickly, potentially endangering the origin/evolution of life on these planets, as liquid water is a key requirement for life. However, Kang (2019b) was looking at planets with obliquities higher than those considered in this work, so moderately high obliquity planet atmospheres may not experience the wetter stratospheres seen in higher-obliquity planet atmospheres. Additionally, potential water loss may be mitigated by the fact that wetter stratospheres only occurred seasonally in simulations from Kang (2019b) and may average out to negligible increased water loss on long-term average for high-obliquity exoplanets and their moderately high obliquity counterparts.

Our results suggest that moderately high obliquity planets may experience both higher export POC production and higher oxygenation potential than their low-obliquity counterparts. However, as we only simulate planetary obliquities up to 45°, it is still unknown how the biospheres on higher-obliquity planets will fare under the physical and climatic changes that occur at higher obliquities. At planetary obliquities greater than 54°, the planet enters a different climate regime where the poles begin to receive more incident insolation than the equatorial regions, potentially leading to the formation of equatorial ice belts (e.g., Kilic et al. 2017, 2018; Rose et al. 2017; Colose et al. 2019). These equatorial ice belts would cause changes to both ocean circulation and surface wind fields, which may have significant effects on biospheric activity. We did not explore these obliquity scenarios because imposing annually averaged winds from ROCKE-3D would likely become an increasingly poor assumption at very high obliquities. Modeling biospheric dynamics and oxygenation potential on planets with higher obliquities remains an opportunity for future work.